This invention relates to integrated circuit analog devices and, more particularly, to a transistor switching circuit for switching an input voltage to an output.
A known JFET switch has been utilized having a three transistor common gate configuration, since this results in a minimum die area during fabrication and high speed characteristics. Three FETs are placed on a common isolated epitaxial island and occupy only fractionally more area than that required by the main switch. The switching speed is increased because of the reduction in the gate capacitance which is a major factor in determining speed.
The prior art approach has the disadvantage that in order to achieve the fast switching characteristics, a large idle current is allowed to flow when the switch is off, thus wasting power and increasing the overall size of the device.
It is therefore an object of this invention to improve the performance of the Tri-JFET switch by reducing the wasted idle power for high speed operation with the resulting reduction in silicon die area.
Briefly, the above object is accomplished in accordance with the present invention by providing a bipolar transistor circuit in combination with the turn-on JFET such that the bipolar transistor turns on only during the transition from open to closed of the switch, and during this transition amplifies the source to drain current of the turn-on transistor to thereby discharge the gate capacitance of the circuit.
The circuit has the advantage that the additional transistor by providing current amplification, makes it possible for the turn-on JFETs to be made small because of their reduced current requirements, without sacrificing any of the high speed switching capabilities of the circuit.
The circuit has the further advantage that it is easy to manufacture because the JFET device and the bipolar device can share one epitaxial island, thus providing considerable area saving on the chip and a sizeable reduction in the common gate capacitance C.sub.g.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.